1. Field of the Invention
The present invention relates generally to fourdrinier paper machines. More specifically, the present invention relates to assembly design and material selection for deckle structures used to confine a papermaking stock pond carried on a fourdrinier screen.
2. Description of the Prior Art
Fourdrinier paper machines are characterized by a closed loop web formation screen driven over an open, flat table surface. Extremely dilute, aqueous papermaking stock is jetted upon the traveling screen from a horizontally elongated nozzle; usually associated with a stock accumulation chamber called a headbox.
As the traveling screen carries the stock flow from the slice jet landing zone, aqueous vehicle, i.e., water, drains through the screen to leave the fiber constituent of the papermaking stock accumulated upon the upper screen surface as a consolidated mat.
Between the stock landing zone and that longitudinally displaced point along the screen belt traveling route whereat the mat consolidates into a paper web, the stock is supported on the screen surface as a liquid pond of diminishing depth. Without lateral containment, lateral liquid stock flow cross-directionally sweeps fiber stock towards the screen sides thereby undesirably tapering the paper web edge thickness.
To prevent such undesirable thickness tapering along the paper web edges, lateral pond confinement structures called "deckle boards" are positioned above and along the screen edges in the machine direction from the slice landing zone. Traditionally, deckle boards are similar to a pair longitudinal dams, each extending along the screen traveling direction respective to each lateral edge of the screen with the screen per se running under the deckle boards.
Elastomer skirts secured to the deckle board rigid structure drag against the underrunning screen for a partial fluid seal. To protect the screen from premature destruction, the elastomer is chosen to be significantly softer than the screen material.
A more recent innovation to the deckle structure has been to combine the deckle board with a screen edge cupping rail located outboard of the deckle board, as represented by U.S. Pat. No. 4,968,387 to R. L. Beran et al. The curled screen edges, traveling along respective, oppositely cupped rail profiles, hydraulically confine the stock pond. The deckle boards, internally of the cupped rails, are vertically positioned above the screen as to leave a substantial hydraulic channel beneath the lower deckle board edge. Machine white water fills the flow channel between the cupping rail and the outside surface of the deckle board. The inside faces of the deckle boards delineate the outer edge limits of the stock fiber. Standing waves generated in the stock pond are permitted to pass under the deckle board into white water channel and dissipate up the edge cup profile without reflection.
Although deckle boards that are operatively combined with screen edge cupping rails do not normally contact the screen, under certain production circumstances, the lower deckle edge is sufficiently close to the screen that frequent contact is inevitable. For this reason, need remains for a soft lower edge for the deckle blade.
Parallel developments have shown that the original generation of formation table deckle waves or waves which may develop from the trailing edge of a deckle blade may be substantially reduced or eliminated by extremely thin and smooth deckle blade construction. Unfortunately, most practical engineering materials that are sufficiently strong and rigid to be usefully thin and smooth are also much harder and tougher than the formation screen material. Consequently, use of thin, smooth deckle blades to eliminate forming table deckle waves has the potential for dramatically reducing the production life of the formation screen.